Primal meat cuts, or primals, are large cuts of meat, smaller, for example, than a side of beef, but larger than the ultimate cut that is sold at retail to the consumer. Primal cuts are prepared at the slaughter house and are then shipped to a retail meat store or an institution such as a restaurant where they are butchered into smaller cuts of meat called sub-primal meat cuts or sub-primals. Sub-primals may also be prepared at the slaughter house. When primals and sub-primals are prepared at the slaughter house, they are usually packaged in such a way that air (i.e., oxygen) is prevented from contacting the meat during shipping and handling in order to minimize spoilage and discoloration. One desirable way to package primals and sub-primals so as to protect them from contact with air is to shrink package them with a packaging material that has good oxygen barrier properties. One such shrink packaging material that has good oxygen barrier properties is polyvinylidene chloride film. However, while polyvinylidene chloride per se has excellent oxygen barrier properties, in actual practice, when polyvinylidene chloride is used as a monolayer film, it must be plasticized in order for the film to have adequate abrasion resistance and flexibility at storage temperatures of, for example, 30.degree. to 50.degree. F. Unfortunately, the addition of sufficient plasticizer, such as from 7 to 9 weight percent of the film, to provide the requisite low temperature properties has a significant adverse effect on the oxygen barrier properties of the film. While increasing the thickness of the film from the conventional thickness of 1.5-2.0 mils to, for instance, 5 mils or more, would improve the oxygen barrier properties of the film, it would be economically undesirable to use a monolayer film of polyvinylidene chloride having a thickness of 5 or more mils. Also, if such thick films were employed, bags made from the film would be difficult to gather and clip at the open end.
One approach to the provision of a film for use in shrink packaging primal and sub-primal meat cuts and processed meats having better oxygen barrier properties than the 1.5 to 2.0 mil monolayer polyvinylidene chloride film previously used for that purpose is to employ a multilayer film, one layer of which is polyvinylidene chloride having a minimum amount of plasticizer. The other layer or layers of such a multilayer film are selected so as to provide the requisite low temperature properties and abrasion resistance which are lacking in polyvinylidene chloride film containing little or no plasticizer. In providing such a film, however, it must be recognized that good oxygen barrier properties, abrasion resistance, and low temperature properties are not the only requirements for a film that is to be used for shrink packaging primal meat cuts. The film must have been biaxially stretched in order to produce shrinkage characteristics sufficient to provide that the film will heat shrink within a specified range of percentages, e.g., from about 30 to 60 percent at about 90.degree. C., in both the machine and the transverse directions. The film must be heat sealable in order to be able to fabricate bags from the film and heat seal the open ends of the fabricated bags. The heat sealed seams of the bags must not pull apart during the heat shrinking operation, the film must resist puncturing by sharp edges such as bone edges during the heat shrinking operation, and there must be adequate adhesion between the several layers of the film so that delamination does not occur, either during the heat shrinking operation or during exposure of the film to the relatively high temperatures that may be reached during shipping and storage of the film in the summertime.
It has been proposed to prepare multilayer films, one layer of which is polyvinylidene chloride copolymer and at least one other layer of which is an ethylene-vinyl acetate copolymer. For example, such films are proposed in McFedries, Jr., et al. U.S. Pat. No. 3,600,267, Peterson U.S. Pat. No. 3,524,795, Titchenal et al. U.S. Pat. No. 3,625,348, and Schirmer U.S. Pat. Nos. 3,567,539 and 3,607,505.
Also in the prior art, cross-linking by irradiation has been used to enhance the properties of films employed in packaging operations. For example, U.S. Pat. No. 3,741,253 to Brax et al teaches a multi-ply laminate having a first ply of ethylene-vinyl acetate which is cross-linked by irradiation. The second ply and the third ply of the laminate are not irradiated. The thus-prepared laminate may then be biaxially stretched. Baird et al U.S. Pat. Nos. 3,821,182 and 4,112,181 teach a three-layer film combination which has been irradiated before stretching. In addition, Kremkau U. S. Pat. No. 4,044,187 teaches irradiating a substrate, forming a film laminate therewith, and then irradiating the entire laminate. Further, Bernstein et al U.S. Pat. Nos. 4,391,862 and 4,352,844 disclose co-extruding first and second polymeric layers, irradiating the co-extruded layers, joining a third layer to the second polymeric layer, and then stretching the multilayer film. Still further, Bieler et al U.S. Pat. No. 4,318,763 teaches that multilayer films may be strengthened by cross-linking one of the layers by irradiation after biaxially stretching. However, the prior art does not teach the concept of irradiating all of the layers of a multilayer film only after biaxial stretching of the multilayer film.
The present invention is based upon the discovery that multilayer films, fully described below, having outer layers of ethylene-vinyl acetate copolymers and having a core layer of a barrier film which are irradiated after biaxial stretching to cross-link the ethylene-vinyl acetate layers, can be successfully employed in the shrink packaging of primal and sub-primal meat cuts and processed meats. Accordingly, this invention provides a multilayer film that can be employed to fabricate bags useful for shrink packaging primal and sub-primal meat cuts and processed meats.